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Photonic Heat Engines: Science and Applications

Prospects for rapid laser cooling on ED-allowed transitions

Alexander J. Salkeld, Stephen Colby Rand, Laura B. Andre, Long Cheng

Published Mar 1, 2019• 1 citation
Publisher

Abstract

We explore candidate materials for solid-state laser cooling on electric-dipole allowed transitions that could accelerate refrigeration. We analyze the required external quantum efficiencies and tolerable impurity absorption required to achieve net cooling, and examine limitations imposed by charge transfer and excited state absorption in Yb2+:SrF2, Ce3+:LiCAF and Ti3+:Al2O3.

DOI
10.1117/12.2508691

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We explore candidate materials for solid-state laser cooling on electric-dipole allowed transitions that could accelerate refrigeration. We analyze the required external quantum efficiencies and tolerable impurity absorption required to achieve net cooling, and examine limitations imposed by charge transfer and excited state absorption in Yb2+:SrF2, Ce3+:LiCAF and Ti3+:Al2O3.
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The potential of three erbium based solids hosts has been investigated for laser cooling. Absorption and emission spectra have been studied for the low lying IR transitions of erbium that are relevant to recent reports of cooling using the 4I15/2-4I9/2 and4I15/2 -4I13/2 transitions. Experimental studies have been performed for erbium in three hosts; ZBLAN glass and KPb2Cl5 and Cs2NaYCl6 crystals. In order to estimate the efficiencies of cooling, theoretical calculations have been performed for the cubic Elpasolite (Cs2NaYCl6 ) crystal. These calculations also provide a first principle insight into the cooling efficiency for non-cubic and glassy hosts where such calculations are not possible.
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Laser cooling of solids has many potential applications in communications, surveillance and medical science, etc. This paper reviews our recent work on laser refrigeration of solids in stoichiometric hexa-chloroelpasolite crystals with high concentrations of erbium. Our results are one to two orders of magnitude improvements over any previously reported cooling in erbium based materials. We have seen bulk crystal cooling by as low as 6 degrees Celsius below room temperature in elpasolite crystals. The high efficiency and low temperatures of cooling have been attributed to a high concentration of erbium, as high as 80% by formula, in the crystal. This cooling has been achieved by pumping near the IR transition at 870 nm which is the weakest transition of erbium. We report cooling in the 1.5 micron transition as well, using a diode laser to pump this transition. The result is cooling by only a fraction of a degree from room temperature.